Field emission displays are desirable for use in viewfinders of video cameras and other display applications requiring low operating power and high pixel density. Such displays typically include an anode electrode coated with an cathodoluminescent phosphor, an array of field emitter tip cathode electrodes, and a perforated grid or gate electrode adjacent the emitter tips. A high positive voltage (e.g., +1,000 volts) is connected to the anode, and a lower positive voltage (e.g., +50 v.) is connected to the grid.
A pixel is the smallest independently-controllable area of the display. Each pixel includes a number of emitter tips which are controlled together. A typical field emission display has an array of tens of thousands of pixels arranged in a matrix of hundreds of rows and hundreds of columns, so that each pixel is uniquely identified by the row and column to which it belongs.
In a raster scan video system, an analog or digital video luminance signal is produced by scanning the brightness of the video image from the left-most column to the right-most column within the top row of the image, then scanning from left to right within the next lower row, and so forth until the bottom row of the image is scanned. At this point the scanning process is repeated. At any instant, the value of the video luminance signal is proportional to the brightness of the pixel at the current scan position.
A field emission display may be "active matrix" or "passive matrix". In an active matrix field emission display, each pixel includes a control circuit connected to the emitter tips in that pixel which controls the voltage at the emitter tips and the current flow through the tips. The emitter tips and control circuit are replicated at each of the thousands of pixels in the display.
In operation, when a pixel is to be dark, that pixel's control circuit raises the voltage at that pixel's emitter tips to a value close to the grid voltage. Conversely, when a pixel is to emit light, the pixel's control circuit reduces the voltage at the emitter tips to a value sufficiently lower than the grid voltage (i.e., the control circuit makes the emitter tip voltage sufficiently negative relative to the grid electrode voltage) so as to cause emission of electrons from the emitter tips. Each emitter tip emits electrons toward the grid, but almost all the electrons are accelerated by the anode voltage so as to pass through apertures in the grid and strike the phosphor coating on the anode, thereby exciting the phosphor to emit light.
An example of an active matrix field emission display is disclosed in commonly-assigned U.S. Pat. No. 5,357,172 to Lee et al., entitled "Current-Regulated Field Emission Cathodes for Use in a Flat Panel Display in Which Low-Voltage Row and Column Address Signals Control a Much Higher Pixel Activation Voltage".
One difficulty in manufacturing field emission displays is ensuring that all of the tens of thousands of pixels have a uniform brightness. The brightness of each pixel is proportional to the pixel current, defined as the total current flow through the field emitter tip cathode electrodes in that pixel. Therefore, a field emission display typically includes a circuit for regulating the pixel current. However, spatial non-uniformities inherent in semiconductor fabrication processes produce pixel-to-pixel non-uniformities in the current provided to each pixel by its associated current regulator.
One circuit for regulating the pixel current in a field emission display is disclosed in the above-mentioned U.S. Pat. No. 5,357,172 to Lee et al. In that circuit, a resistor in each pixel control circuit determines the pixel current flow, so that the pixel-to-pixel uniformity of the current depends on the pixel-to-pixel uniformity of the resistance values of the resistors. Because the pixel current in a field emission display typically is in the range of only 0.1 to 300 nA, the resistors must have resistance values on the order of 10.sup.8 to 10.sup.11 ohms. Unfortunately, it is difficult to manufacture such high value resistors with the desired uniformity.
U.S. Pat. No. 5,396,150 to Wu et al. discloses a field emission display employing a separate resistor in series with each field emitter tip for the stated purpose of "obtaining emission uniformity by sustaining the cathode to gate voltage". Because Wu has one resistor per emitter tip rather than one resistor per pixel as in Lee et al., Wu's resistors each conduct less current than Lee's for a given pixel brightness, hence must have even higher resistance than Lee's. Consequently, Wu's resistors are subject to the same or worse non-uniformity problems as those in the Lee et al. disclosure.
The PhD thesis "Current Limiting of Field Emitter Array Cathodes" by Kon Jiun Lee (Georgia Institute of Technology, August 1986) also discloses a field emitter array having a resistor in series with each field emitter tip for the purpose of improving emission uniformity. The resistors having values on the order of 10.sup.8 to 10.sup.9 ohms were fabricated of undoped, intrinsic silicon. The thesis also proposed that a reverse-biased p-n junction diode, either alone or in combination with a resistor, was expected to be superior to a resistor alone if a p-n junction could be successfully fabricated which could tolerate practical current and temperature levels without reversible breakdown. The thesis proposed fabricating the diode of Ge, PbS, or InSb because a silicon diode would have too low a reverse-biased saturation current.
Three other circuits for regulating the pixel current in a field emission display are disclosed in U.S. Pat. No. 5,162,704 to Kobori et al., entitled "Field Emission Cathode". FIGS. 1 and 2 of Kobori show a reverse-biased Schottky diode connected in series between each emitter tip and a cathode electrode to regulate the pixel current. FIG. 3 of Kobori shows a current regulator circuit having a field-effect transistor (FET) and a resistor which can be substituted for the Schottky diode. Undesirably, the designs of Kobori's FIGS. 1-3 seek to maintain a constant current in each emitter tip, thereby making the designs unsuitable for displays which vary the amplitude of current in each pixel in response to a video luminance signal. (That is, the designs could be used only in displays which vary the duty cycle, rather than the amplitude, of the pixel current in response to the luminance signal.) In addition, the FET/resistor design in Kobori's FIG. 3 has the same disadvantage as U.S. Pat. No. 5,357,172, namely, that the pixel current uniformity depends on the uniformity of resistors in the Gigohm range. Another design shown in FIG. 4 of Kobori requires bipolar transistors, which would be undesirable for use in a display which uses field-effect transistors in other parts of the display circuit, because fabricating both types of transistors on a substrate typically requires a more complex fabrication process with additional fabrication steps.